Question

a3=3, a5=33. d=? S20=?​

Answer 1

Ответ:

d = 15

S₂₀ = 2310

Объяснение:

$a_{n+1} = a_{n} +d$

a₄ = a₃ + d

a₅ = a₄ +d

a₅ = a₃ + 2d

d = (a₅-a₃)/2 = (33-3)/2 = 15

a₁ = a₃ - 2d = 3 - 30 = -27

$S_{n} = \frac{2a_{1}+(n-1)d }{2} n$

$S_{20} = \frac{2*(-27) + 19*15}{2} *20 = 10*(285-54) = 2310$